Inflammation is an established risk factor for diseases that are common with age, including cardiovascular and autoimmune diseases, type II diabetes, certain cancers, and functional and cognitive impairment. Stress is linked to elevated inflammation and dysregulation of related physiological systems, including the hypothalamic- pituitary-adrenal (HPA) axis. In contrast, positive aspects of well-being (such as positive emotions) are associated with better health and longevity, but their roles in shaping biological processes and their interactions with stress are not well understood. The goal of this research is to examine whether positive experiences in everyday life protect against dysregulation of inflammatory processes and diurnal cortisol rhythms (a measure of HPA axis functioning and modulator of inflammation). The proposed research will use data from an existing study of 300 adults, ages 25- 65, who will complete 4 bursts of data collection spaced approximately 8 months apart. Each burst includes 14 days of smartphone-based momentary assessments collected 4 times per day to capture mood, positive events, and stressors. During each burst, saliva samples are collected 4 times per day for 5 days to assess cortisol. Blood is collected at the end of each burst to assess circulating inflammatory biomarkers and stimulated inflammatory responses (ex vivo to an immunologic stressor). The associations of daily well-being (i.e., momentary positive affect and positive events) with inflammatory state, inflammatory responsiveness, and diurnal cortisol will be modeled within- and between-persons, and across different time scales ranging from days to several years. This study will also test the extent to which daily well-being attenuates the effects of acute and chronic stress on physiological functioning. The proposed research has implications for the development of strategies to promote healthy aging and to improve mental and physical health. The research training program is designed to build the applicant's expertise in four areas: (1) lifespan perspectives on emotions, health, and aging; (2) biological processes involved in stress and aging; (3) state-of- the-art statistics and methodology for multilevel, intensive longitudinal designs; and (4) manuscript- and grant- writing and professional development. Training will involve a variety of activities, including coursework, colloquia and seminars, directed readings, and hands-on laboratory and statistical training. This fellowship will prepare the applicant for a career as an independent research scientist focusing on biological pathways that link psychological well-being to long-term health outcomes.